In epistemology and the philosophy of perception, phenomenalism is the view that physical objects do not exist as things in themselves but only as perceptual phenomena or sensory stimuli (e.g. redness, hardness, softness, sweetness, etc.) situated in time and in space. In particular, phenomenalism reduces talk about physical objects in the external world to talk about bundles of sense-data.

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Phenomenalism is a radical form of empiricism and, hence, its roots as an ontological view of the nature of existence can be traced back to George Berkeley and his subjective idealism. As an epistemological theory about the possibility of knowledge of objects in the external world, however, it is probable that the most perspicuous formulation of phenomenalism is to be found in the transcendental aesthetics of Immanuel Kant. According to Kant, space and time, which are the a priori forms and preconditions of all sensory experience, "refer to objects only to the extent that these are considered as phenomena, but do not represent the things in themselves". While Kant insisted that knowledge is limited to phenomena, he never denied or excluded the existence of objects which were not knowable by way of experience, the things in themselves or noumena, though he never proved them.

Kant's "epistemological phenomenalism", as it has been called, is therefore quite distinct from Berkeley's earlier ontological version. In Berkeley's view, the so-called "things in themselves" do not exist except as subjectively perceived bundles of sensations which are guaranteed consistency and permanence because they are constantly perceived by the mind of God. Hence, while it is true that for Berkeley, objects are merely bundles of sensations (see bundle theory), unlike other bundle theorists, objects do not cease to exist for Berkeley when they are no longer perceived by some merely human subject or mind.

In the late 19th century, an even more extreme form of phenomenalism was formulated by Ernst Mach, later developed and refined by Russell, Ayer and the logical positivists. Mach rejected the existence of God and also denied that phenomena were data experienced by the mind or consciousness of subjects. Instead, sensory phenomena, for Mach, are "pure data" whose existence is to be considered anterior to any arbitrary distinction between mental and physical categories of phenomena. In this way, it was Mach who formulated the key thesis of phenomenalism and that which separates it from bundle theories of objects: objects are logical constructions out of sense-data or ideas.

Phenomenalism is frequently confused with the bundle theory of perception and vice-versa. According to the bundle theory, objects are made up of sets, or bundles, of ideas or perceptions. To say that the pear before me exists is simply to say that certain properties (greenness, hardness, etc.) are being perceived at this moment. When these characteristics are no longer perceived or experienced by anyone, then the object (pear, in this case) no longer exists. Phenomenalism is the view that objects are logical constructions out of perceptual properties. On this view, to say there is a table in the other room when there is no one in that room to perceive it, is to say that if there were someone in that room, then that person would perceive the table. It is not the actual perception that counts, but the conditional possibility of perceiving.

The idea of the logical positivists, such as A.J. Ayer and Bertrand Russell, was to formulate the doctrine of phenomenalism in linguistic terms, so as to define references to such entities as physical objects in the external world out of existence. Sentences which contained terms such as "table" were to be translated into sentences which referred exclusively to either actual or possible sensory experiences. Roderick Chisholm definitively refuted this version of phenomenalism in 1948.

To see how he did this, note that C.I. Lewis suggested that the physical claim "There is a doorknob in front of me" necessarily entails the sensory counterfactual "If I should seem to see a doorknob and if I should seem to myself to be initiating a grasping motion, then in all probability the sensation of contacting a doorknob should follow." Of course, this statement itself contains references to physical objects which would have to be substituted by sense-data expressions, but the point is clear enough. Chisholm showed that the statement "There is a doorknob..." does not entail the counterfactual statement. If it were to do so, then it must do so without regard to the truth or falsity of any other statement. But suppose the following statement is true: "I am paralyzed from the neck down and experience hallucinations such that I seem to see myself moving toward the door". If this is true, then there could be a doorknob in front of me, I could seem to myself to see a doorknob, and I could seem to myself to be performing the correct sort of grasping motion but with absolutely no chance of having a sensation of contacting the doorknob. Likewise, the statement that "The only book in front of me is red" does not entail the sensory statement "Redness would probably appear to me were I to seem to myself to see a book" because redness is not likely to appear under a blue light-bulb.

Some have tried to avoid this problem by extending the conditions in the analysandum: instead of "There is a doorknob in front of me" one could have it that "There is a doorknob...and I am not paralyzed, etc.."
But if one complicates the analysandum, one must also complicate the analysans. In this particular case, one must analyse in purely sensory terms what it means not to be paralyzed and so on. The same problems would arise with respect to the new analysis and we would have an infinite regress.

Another common objection to phenomenalism is that in the process of eliminating material objects from language and replacing them with hypothetical propositions about observers and experiences, it seems to commit us to the existence of a new class of ontological object altogether: the sensibilia or sense-data which can exist independently of experience. Indeed, sense-data have been dismissed by some philosophers of mind, such as Donald Davidson, as mythological entities that are more troublesome than the entities that they were intended to replace.

A third common objection in the literature is that phenomenalism, in attempting to convert propositions about material objects into hypothetical propositions about sensibilia, postulates the existence of an irreducibly material observer in the antecedent of the conditional. In attempting to overcome this, some phenomenalists suggested that the first observer could be reduced by constructing a second proposition in terms of a second observer, who actually or potentially observes the body of the first observer. A third observer would observe the second and so on. In this manner we would end up with a "Chinese box series of propositions" of ever decreasing material content ascribed to the original observer. But if the not final result is not the complete elimination of the materiality of the first observer (which it cannot be), then the translational reductions that are proposed by phenomalists cannot, even in principle, be carried out.

A final, and perhaps the most devastating objection, to phenomenalism was formulated by R. Firth (1950). The objection stems from perceptual relativity: white wallpaper looks white under white light and red under red light, etc.. Any possible course of experience resulting from a possible course of action will apparently underdetermine our surroundings: it would determine, for example, that there is either white wallpaper under red light or red wallpaper under white light, and so on. On what basis are we to decide which of the hypotheses is the correct one if we are constrained to rely exclusively on sensibilia?